Manufacture of disodium phosphate



Patented Nov. 8, 1927.

- UNITEDg-STATES PATENT OFFICE.

HERBERT H. MEYERS, OF PITTSBURGH, PENNSYLVANIA, ASSIGNOR TO ARMOUR FER-TILIZER WORKS, OF CHICAGO, ILLINOIS, A CORPORATION OF NEW JERSEY.

MANUFACTURE OF DISODIUM PHOSPHATE.

No Drawing.

Disodium phosphate as it at present universally appears on the market isin the form of a white, pure, crystallized SOlld containing twelvemolecules of water of crystallization, its formula being tallization isdriven off by heating, and, in

fact, the formation of sodium pyrophosphate starts at slightly over 98centigrade, and, so far as is known, no one has succeeded in preparing,by heating, a dehydrated disodium orthophosphate without at the sametime producing at leasttraces of.

the pyro salt. I

The usual process of preparing disodium orthophosphate (Na HPO 12H O) isto treat dilute phosphoric acid, either pure or impure, which contains asuficient excess of water, with sodium carbonate 1n the right proportionto produce the disodium salt, whereupon this solution is filtered, ifnecessary, to remove solid impurities, and then the Na POJQH O isallowed to crystallize out in various crys tallizing devim, the crystalsbeing washed and then filtered oil and dried, whereupon there isobtained the ordinary disodium phosphate of commerce containing but19.9% P 0 This pantentee has prepared a disodium phosphate in a verysimple manner, which contains very materially more P 0 content than theNaPO,.12H O,

as it contains less water of crystallization, and in some casespractically no water of crystallization, and at the same time it con-Application filed January 14, 1927. SeriaI No. 161,241.

tains no sodium pyrophosphate, unless it is subsequently heated toincrease still further the P 0 content, when it also may contain meretraces of sodium pyrophosphate. I

This product may be prepared pure, or, 1n many cases, it may .besatisfactorily placed on the market containing impurities present in thephosphoric acid, which are in many instances harmless for the purposesto which the disodium phosphate is put in commerce.

The general method by which this materlal is prepared by my improved andnovel process is as follows:

Phosphoric acid of as great concentration as will permit of mixing inthe required amount of'soda ash is used, the actual concentrationvarying with the purity and temperature of the phosphoric acid, but, inany case, it will contain considerably less water than is necessary toform the na rrronen o and to this concentrated phosphoric acid there isadded, in the form of a powder or dust, the required amount of soda ashto form the disodium salt, and the mixture, which'forms a hot mass ofrather high viscosity, and, in case the acid contains a certain amountof impurities which precipitate, is a rather thick slurry, is thoroughlystirred so that the reaction is completed.

This hot mass is then run into pans Where, after standing awhile, as itcools 1t crystallizes into a solid mass, which may be somewhat porousdue to the presence of CO generated by the reaction of the acid and sodaash. i

There may also be present some undecomposed soda ash, if a little excessof this is employed, or the reaction has not quite been completed.

Such solidified mass is thereupon removed from the pans, or it may beloosened from the pans while it is somewhat hot to make its removaleasy, and it is then ground into any size particles it is desired tomake, by

any of the usual methods and apparatus for grinding. 1

This improved and novel process of producing disodium phosphate has anumber of marked advantages, since it gives a higher salt content thanthe usual method employed, it is cheaper to prepare and furnishes amaterial which is .not necessarily pure, where a pure product is notrequired, hence obviating the necessity of removing impurities, andconsequently cheapening the lwproduct may be completely dehydrated byeating just as in the usual method, in which case there probably will bepresent a mere trace of the pyrophosphate.

The preferred method of preparing the salt referred to is as follows:

A charge consisting of twelve hundred parts by weight of commercialphosphoric acid of about 60 Baum, such as is shipped ordinarily incommerce, containing about 53 to 54%by weight of P 0 is introduced intoa wooden stirring tank provided with a pad dle, and to this is added aslight amount of water, say, between two hundred and four hundred partsby weight, depending on what analysis of the final product is desired,and then, with constant stirring, one thousand parts by weight of sodaash are added.

This mass is then thoroughly stirred, there is a considerable evolutionof CO and the precipitation of the impurities in the phosphoric acidoccurs.

Such mass becomes hot and thickens considerably, so that in order toassure that all of the lumps are well broken up, the charge isdischarged, continuously through a small aste mill such as is used inthe paint inustry, and is delivered directly into an iron pan, in whichit is allowed to cool 0H and become crystallized.

It is well, however, for the sake of ease and facility in removing itfrom the pan, to loosen it somewhat from the bottom while the mass isstill slightly soft, and upon such removal, in the form of hard,crystallized material, it is introduced into a cage mill or hammer milland ground to whatever fineness is desired.

In this way, a product is economically produced containing between 30%and 40% P 0 depending upon the amount of water added to the phosphoricacid at the start.

The product may contain a slight deficiency of sodium below thatrequired to form the disodium salt, or it may contain an excess of sodaash to any degree preferred, but this depends on the use to which theproduct is to be put and aflects in no zvay the principle of the methodof prepara- If pure phosphoric acid has been used to prepare this salt,the analysis, of course, will be higher than if the product contains theimpurities originally present in commercial phosphoric acid, but, formany of the uses of disodium phosphate, such as boiler-water treatment,these impurities are of no consequence, and the commercial acid may beused to'advantage.

The product is, therefore, a disodium phosphate containing less thantwelve mole cules of water of crystallization and which may have presentin it the impurities present in the materials from which it is prepared;and the process by which such material is produced is the treatment ofphosphoric acid of such concentration that in combination with therequired amount of soda ash and,

subsequent cooling, it will set to a solid mass which can bedisintegrated and shipped for use in the industries,the concentration ofthe acid varying considerably, depending on the product which it isdesired to produce, in regard to the content of P 0, and the degree ofpurity required.

I claim:

1. In the process of making disodium phosphate, mixing togetherphosphoric acid, a sodium salt of an acid more volatile thanphosphoric'acid, and less water than is necessary to form and permittingthe product to cool and harden.

2. In the process of making disodium phosphate, mixing togetherphosphoric acid, soda ash, and less water than is necessary to form NaHPO,.12H,O,

and cooling the product in a pan.

3. In the process of making disodium phosphate, mixing togetherphosphoric acid, a sodium salt. of an acid more volatile than phosphoricacid, and less water than is necessary to form Na HPO,.12H O,

cooling the product in a pan, and loosening it from the pan during thecooling operation.

4. In the process of making disodium phosphate, mixing togetherphosphoric acid, soda ash, and less water than is necessary to form NaHPO,.'12H,O,

phoric acid of about 60 Baum and containing in the neighborhood of 53%by weight of ing in the neighborhood of 53% by weight of P about 200 to400 parts by weight of P 0 about 200 to 400 parts by weight of water,and approximately 1000 parts by water, and approximately 1000 parts byweight of soda ash, passing the resulting 5 weight of soda ash,discharging the mass mass through a paste mill and dischargingcontinuously through a paste mill, and deit into a pan for cooling andcrystallization, livering it into a pan for cooling and cryslooseningthe mass from such pan While it tallizatlon. is still slightly soft, andgrinding the hard 7. In the process of making disodium crystallizedmaterial to the required fineness. l0 phosphate, mixing togetherapproximately In witness whereof I have hereunto set my 1200 parts byWeight of commercial phoshand. phoric acid of about Baum and contain-HERBERT H. MEYERS.

